internal/core/adt: add tests for issue 3837

Issue #3837

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: Idafb2f9791e488a364ffddcedb44b9f213bb90af
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1211950
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mpvl